Opening/closing assisting devices for loading deck side plates are generally of a structure wherein a spring mounted on the underside of a loading deck and a side plate are connected together with a link mechanism mounted on the underside of a side end of the loading deck. The spring bears a portion of the weight of the side plate so that the side plate can be opened and closed lightly. The link mechanism changes the direction of the spring force into approximately vertical direction so as to apply the force to the side plate from approximately a horizontal direction. And at the same time it changes the magnitude of the force according to the opening angle of the side plate. In other words, the link mechanism functions so that the assisting force is approximately zero when the side plate is in a full open or closed state and becomes maximum when the side plate is opened up to a substantially horizontal position. Usually, the link mechanism is constituted mainly by a bell crank (see, for example, Japanese Patent Laid Open No. 67570/83).
Consequently, the link mechanism portion becomes large-sized and particularly its vertical size becomes larger than that of the spring portion, resulting in that when used in a low floor vehicle, the link mechanism portion interferes with a fuel tank mounted on the underside of the loading deck and thus the mounting thereof causes many difficulties.
There is also known a construction using wire and pulley for changing the direction of force (see Japanese Patent Laid Open No. 129026/76). According to this construction, it is possible to make the vertical size smaller than that using a bell crank, but the durability is unsatisfactory because wire is used and it is impossible to change the magnitude of force by the pulley.